A spectrometer is an apparatus for measuring a spectrum of electromagnetic radiation such as light. Spectrometers are widely used in science and industry as powerful analytical and measurement tools. For example, they are used for remote sensing of temperature, determining chemical composition and concentration of chemical compounds, and identifying substances.
Spectrometers for measuring optical power as a function of wavelength are called optical spectrum analyzers (OSAs). Most conventional OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or a diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle dependent on the wavelength. The spectrum of light is then analyzed on the basis of the angle at which the light is diffracted using a detector array. Alternatively, the diffracted light is moved over a slit and then detected using a photodetector.
Traditional OSAs are manufactured as laboratory devices, which operate under laboratory environmental conditions. A sophisticated wavelength and optical power calibration is required from time to time to ensure the wavelength and power accuracy of the devices. Furthermore, they are generally bulky as well as costly.
Optical communication systems employing wavelength division multiplexing (WDM) technology achieve large transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm) apart. As the channel spacing decreases, monitoring spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. For example, such monitoring is critical in detecting wavelength drift, which can readily cause signals from one optical channel to cross one into another. Also, real-time feedback to network elements is critical to ensure stable operation of optical amplifiers commonly employed in the network.
Optical communication systems require industrial grade optical performance monitors (OPMs), which function similarly to the traditional OSA, but are, however, subject to stringent industrial requirements. They must be relatively inexpensive, compact in size, with the reporting power and wavelength accuracy nearly the same as laboratory grade OSA, however without requiring extra calibration during the lifetime of the device, and be capable of monitoring light at densely spaced frequency points with high spectral resolution and high dynamic range. An OPM usually outputs a spectrum as a function of optical frequency rather than wavelength, because the standardized spectral grid of optical channels, the so-called ITU grid, is equidistant in frequency rather than the wavelength units.
It is advantageous to have an OPM capable of monitoring all channels in one optical band of an optical communication link. It is also advantageous to have an additional functionality of monitoring an optical to signal noise ratio (OSNR) for each channel, which requires monitoring not only individual channels, but also light between channels to estimate an optical noise level, thereby further increasing spectral resolution requirements for an OPM. Today's WDM networks may employ as many as 200 channels with 25 GHz spacing between the channels in one optical communication band of approximately 5000 GHz; these networks would benefit from an OPM capable of monitoring at lest 200 frequency channels with 25 GHz spacing. Such an OPM could also be advantageously used in communication systems having 200 GHz, 100 GHz, and 50 GHz spaced channels by providing an OSNR monitoring capability.
One type of industrial-grade OPM acquires the spectrum by angle-tuning a dispersive element such as diffraction grating. For example, U.S. Pat. No. 6,118,530 by Bouevitch et al. assigned to JDSU corporation, which is incorporated herein by reference, teaches a scanning frictionless spectrometer with magnetically actuated flexure-supported diffraction grating and a dedicated separate channel for accurate wavelength referencing during each scan. The advantage of the scanning approach of Bouevitch et al. is based on the ability to continuously sweep the wavelength across the entire spectral region, which greatly improves fidelity of spectra obtained, as well as accuracy of OSNR and peak wavelengths determination. Detrimentally, a scanning spectrometer is often slower than its detector array based counterpart. A slower measurement speed results from the fact that, in a conventional scanning spectrometer, most of incoming light is discarded, and only a narrow optical frequency component is allowed to impinge on a photodetector at any given time. Moreover, having to rotate a relatively large optical element such as diffraction grating reduces overall reliability and expected lifetime of a conventional scanning OPM.
Another type of industrial-grade OPM acquires all monitored spectral points of an optical spectrum of an input signal in parallel by dispersing the input light in space and using a plurality of photodetectors, e.g. a photodiode array (PDA), to simultaneously acquire spectral information at a plurality of monitored frequencies; a bulk grating, a blazed fiber Bragg grating, a waveguide echelle grating or an array waveguide grating (AWG) can be used as a dispersive element.
Disadvantageously, the number of photodiodes in the PDA scales proportionally to the required wavelength resolution, thereby increasing the size and cost of the device and reducing its reliability. If the OSNR of each channel is to be measured, several photodetectors have to be provided within the dispersed light of a single channel. Thus, a four channel optical monitor typically requires at least 12 photodiodes. Since current photodiode arrays are often supplied in strips of up to 128 photodiodes, this allows monitoring of just over 30 channels.
U.S. Pat. No. 5,617,234 issued Apr. 1, 1997 in the name of Koga et al., which is incorporated herein by reference, discloses a multi-wavelength simultaneous monitoring circuit capable of precise discrimination of wavelengths of a WDM signal, and suitable for optical integrated circuits. The device proposed by Koga is an arrayed waveguide grating (AWG) that has a single input port and multiple output ports and has photodetectors coupled to the output ports of the AWG. Koga's device requires an AWG having a number of output ports equal to a number of monitoring channels with frequency resolution better than spacing between the channels, and a number of costly photodetectors equal to the number of channels to be monitored, without providing an OSNR measurement capability.
Recently, attempts have been made to provide a solution to the problem of scaling by combining the aforedescribed sequential and parallel acquisition approaches in a way wherein the size, the design complexity, e.g. the number of photosensitive elements, and the control complexity of the monitor scale sub-linearly with a number of monitored wavelengths within a monitored range of wavelengths.
For example, U.S. Pat. No. 6,753,958 in the names of Berolo et al., which is incorporated herein by reference, discloses an approach to monitoring a large number of wavelengths with a relatively small number of photodiodes without dynamically tuning wavelength-selective elements that may require complex real-time monitoring and control. Berolo et al. teach an OPM that has an optical input port coupled through a switch to a plurality of input waveguides, which are sequentially switched to provide light received from the input port via one of the input waveguides to a waveguide echelle grating, which disperses the light toward a plurality of photodetectors. The echelle grating disperses light received from an input waveguide in dependence upon the input waveguide position, so that light picked up by the photodetectors has different centre wavelengths depending on via which of the input waveguides the light entered the grating. By arranging the input waveguides so that the centre wavelengths sampled by photodiodes shift by a fraction of the channel spacing of the WDM signal when the light is switched between adjacent input waveguides, the WDM signal carried by the light can be sampled with a frequency period equal to the fraction of the channel spacing. Disadvantageously, when the optical channel density is increased, the crosstalk between the input waveguides effectively limits amount of data points available.
Further, U.S. Pat. Nos. 7,035,505 and 7,130,505 by Shen et al. assigned to JDSU corporation, which are incorporated herein by reference, disclose an optical performance monitor based on a demultiplexing arrayed waveguide grating (AWG) having a plurality of Vernier input waveguides disposed between an optical switch and a photodiode array coupled to the output waveguides of the AWG. A specific position pattern of M input waveguides having a frequency spacing different from the frequency spacing of the N output waveguides in a manner similar to a well-known Vernier scale, e.g. a scale in Vernier calipers, serves to provide a possibility to measure a spectrum at up to M×N frequency points. Detrimentally, the approach of Shen et al., due to Vernier geometry, produces a set of folded spectra and therefore requires computation-intensive spectral unfolding processing. Further, for both approaches of Berolo et al. and Shen et al., an external waveguide-coupled optical switch is required which complicates the design and increases optical losses.
It is an object of the present invention to provide a spectrometer combining the advantages of both scanning and detector array-based approaches. The spectrometer of the present invention provides a high-fidelity, scanned optical spectrum, with a number of measurement points not limited by the apparatus geometry. Further, advantageously, the spectrometer of the present invention uses parallel detection, with much less light being discarded for spectral selectivity than in a prior-art scanning approach described above. Yet further, advantageously, the spectrometer of the present invention allows for multiple selectable input ports without an associated increase in complexity, e.g. without having to incorporate an additional switch at the input, which leads to significant cost savings for a system requiring multiport spectral measurements.